VSPAero ground effect distance origin?

[skyc...@gmail.com]

Hi,

Is the ground effect distance used by VSPAero calculated relative to the z_cg value in VSPAero, or relative to the OpenVSP model coordinate system?

Thanks,

Bryan

[Brandon Litherland]

It's relative to the Moment Reference Position, Zref.

I just did a quick search of the source code and the comments suggest this is true. https://github.com/OpenVSP/OpenVSP/blob/7da9d8c7e7137e0680a98fdf9ace500fc2c5f323/src/vsp_aero/Solver/vspaero.C#L485

I also checked VSPAERO by setting ground height to 2.0 and the CG height to zero.  With a default wing placed at Z = 0, the CL is about 0.83 for 10 degree alpha and Mach 0.1 / Re 1e6.  If I then move the wing to Z = 5 and run again without changing the ground height or CG height, the CL drops to about 0.78.  If I then change the CG height to 5 (wing Z and CG Z delta is back to zero) and run again, I get the original 0.83 CL.

So the ground effect is defined relative to the CG_ref and will apply it appropriately to all geometries and their relative distances to the CG.  I did some more digging into the code and found where VSPAERO applies ground effect height.  The calculation is made to change the Z position of every vortex plate in the system.  So when Dave warns in the code not to rotate your geometry into the ground, he expects you to know how that could happen.

https://github.com/OpenVSP/OpenVSP/blob/7da9d8c7e7137e0680a98fdf9ace500fc2c5f323/src/vsp_aero/Solver/VSP_Surface.C#L5809

[skyc...@gmail.com]

Thanks!

One follow up question, can you do alpha sweeps using the ground effect and it will calculate it appropriately, or do you have to physically rotate the geometry?

Thanks,

Bryan

[Jorge Andres Piñero Delgadillo]

Hello Bryan

I have run several ground effect analyzes, and just by adjusting the alpha, the results are fine without having to rotate the geometry.
Still, the results manager does not show alpha sweeps when the ground effect toggle is on. You need to do one sweep per time, varying the ground effect distance and alpha degree.

Hope this helps.

Piñero D. Jorge A.

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